Within the field of electronic radio communication devices, there is a desire to reduce the overall size of such devices. Additionally, the reduction in size of electronic components recently has allowed the size of printed wiring board (PWBs) to be reduced.
Antenna arrangements for radio communication devices usually include unbalanced resonant antennas which require a ground plane to operate. In most devices, the printed wiring board acts as the ground plane for the antenna elements. If the largest dimension of the ground plane is of the order of λ/2 or a multiple of λ/2 (where λ is equal to the operating wavelength), the ground plane can also support radiating resonant modes of its own. At radio communication frequency bands (850 MHz for example), miniaturization of antenna elements can be achieved by using the antenna elements not only as radiators but also to excite resonant modes of the ground plane which then radiates a significant portion of the signal from the device.
In order to maximise the operational bandwidth of a relatively small antenna element on a portable radio communication device, resonant frequencies of the resonant modes of the antenna and the ground plane should be substantially equal and there should be relatively strong coupling between the resonant modes. The lowest order mode of the ground plane resonates when its largest dimension (usually the length) is equal to λ/2. Antenna elements can affect the electrical length of the ground plane, making it either electrically longer or shorter than the physical length of the ground plane. Further ground plane resonances occur when the electrical length of the ground plane is a multiple of λ/2. The optimal ground plane lengths (or other dimensions) for different frequencies can be found using characteristic mode analysis, for example.
The electrical length is the length of a current path expressed in terms of the wavelength. The electrical length may be related to the physical length of the ground plane for longitudinal resonant modes or the width of the ground plane for transverse resonance modes. The electrical length need not be equal to any of the physical dimensions, as for example meandering or adding discrete components change the electrical length. In addition, adding a slot in the ground plane makes the electrical length longer as the current path is a combination of transverse and longitudinal components. A device will usually have multiple electrical lengths as different antennas generate different current distributions and resonance modes at the various operating frequencies.
As the size of the printed wiring board is reduced (below 100 mm for example), the performance of the antenna arrangement may be worsened due to the printed wiring board having an electrical length which is too short for the desired operational frequency band. Consequently, it may be difficult to achieve reasonable antenna performance in a relatively small device.
It would therefore be desirable to provide an alternative apparatus.